A particular advantage of the open end spinning of yarns over their production by "ring" spinning machines is speed, in which in identical time intervals open end machines can produce from three to ten times the amount of yarn as can ring spinners.
In efforts to increase this advantage, it has been proposed to increase the speeds of rotation of the yarn spinning rotor from a presently conventional 30,000 to 45,000 revolutions per minutes (r.p.m.) to speeds to excess of 55,000 r.p.m.
One problem encountered in such attempts has been that of retaining the rotor cup securely fastened to its supporting shaft or spindle, despite the extensive forces imposed at such very high rotational speeds. Among the these forces is the very great centrifugal force imposed upon the rotor cup and its interconnection with the shaft, which force increases with rotational speed and imparts bending and other distortional effects on these members.
Among methods conventionally employed to secure the rotor to its supporting shaft are fastenings through the use of detachable elements such as screws and the force fitting or "interference" fitting of the shaft into a bore of a terminal boss of the rotor.
While screw fastenings, such as are disclosed in German patent application (Offenlegungsschrift) No. 2441846 and British Pat. 1,410,972, as a concept are attractive in the promise of permitting the facile and rapid exchange of one rotor cup for another on the support shaft, such as may be required for example when one wishes to change the size of yarn to be produced or when a worn rotor cup is to replaced by an unworn cup, such fastenings give rise to the real problem of loosening in time during processing of fibers to yarn. In assembly of the rotor cup to the shaft, the connecting screw has to be tightened. The efficacy of this fastening resides not only in the amount of force used in the tightening and its consistent application from one rotor to shaft combination to another in the machine, but also in the strengths of the various joined components and their materials of construction, such as the screw itself, and its screw thread, and the female screw thread of the shaft as well as the resistance to distortion of the conjoined cup and shaft when tightening forces are applied. Further, the efficacy of fastening must be maintained throughout the yarn processing period when the shaft, cup, and conjoined screwthreads are subjected to the high forces previously mentioned. In all, screwthread fastenings raise severe doubts as to their efficacy, and especially so in efforts to raise the rotational speeds of rotors to higher and even higher levels.
In recognition of the foregoing, many commercial open end spinners employ the force or interference fit type of conjoining of rotor to supporting shaft. The prior art discloses many types of rotors so conjoined with their supporting shafts, such as in for example British No. 1,170,869; 1,200,355 and 1,228,358. More recent disclosures can be found in U.S. Pat. Nos. 4,068,456; 4,058,963; 4,022,011; 4,008,561; 3,981,132; 3,975,895; 3,973,382; and 3,972,171, for example.
Observation of force fit coupled rotor cups and shafts at high rotational speeds such as in excess of 55.000 r.p.m. discloses significant distortions at these speeds of the cup and its boss relative the supporting shaft, significant to the extent of reducing the grip of the boss on the shaft and permitting the cup and boss to be displaced on the shaft both axially and longitudinally. It has been found that at such high speeds there is a tendency for the bore through the boss to "expand" due to the effects of centrifugal forces acting upon the cup portion and the material of the boss, resulting in a loosening of the boss on the shaft. In such circumstances there is the danger that the rotor so loosened and displaced may contact adjacent elements such as, for example, the doffing tube which projects into the cup cavity, and the further danger that such displacement may be to the extent that the rotor may become completely detached from its shaft.